1. Field of the Invention
This invention relates to a piston ring for internal combustion engines. In particular, the invention relates to a piston ring that prevents aluminum plating on the flank surfaces of the piston ring when used in conjunction with aluminum pistons.
2. The Prior Art
Piston rings for internal combustion engines must satisfy three requirements: First, they must seal the combustion chamber against the crankcase. Second, they must limit the consumption of oil, and third, they must dissipate the heat absorbed by the bottom of the cylinder into and through the wall of the cylinder and into the coolant.
The piston ring can seal the combustion chamber only if a continuous line of contact permanently exists between the surface of the cylinder and the running surface of the ring, except for the point of play of the joint. In addition one of the two flank surfaces of the piston must abut the corresponding surface of the piston-ring groove in an air- or gas-tight manner. This is accomplished if the piston ring is compressed against the cylinder wall by the action of the gas forces developed by the build-up of pressure between the inner jacket surface of the ring and the bottom of the groove. The piston rings are seated in the piston-ring groove with very little play. However, they are capable of performing axial, radial and rotary motions and tilting in the groove. Such movements are caused by the forces of inertia acting as a result of the piston movements, and by the combustion pressure. This may cause the flanks of the ring to wear and erode.
Many different kinds of piston ring materials are used in modern high-performance engines. These materials are subjected to various surface treatments and hardening methods in order to counteract the wearing phenomena. A common application for minimizing wear at low cost is the use of piston rings that have been nitrated on all sides. The basic material may be a steel with a high chromium content, or cast metal. These piston rings have the drawback that when they are used with aluminum pistons, they tend to cause plating with the aluminum. This plating occurs mainly on the lower of the piston ring flank surfaces, i.e., on the surface facing away from the combustion pressure. In addition, temperature and prevailing lack of lubricant has a significant effect, especially on the uppermost compression ring.
German Patent DE 35 38 978 C1 discloses a method for reducing static friction of a piston ring in the piston ring groove. This method involves relieving the pressure on the lower flank of the piston ring by guiding the gas pressure through bores or channels into a flute in the lower piston ring side surface. U.S. Pat. No. 4,438,937 and French Patent No. 2 699 600 both disclose measures for inertia reduction and weight relief through recesses in the side surface of the piston ring. However, these prior methods have the drawbacks that they require costly additional work and do not eliminate the risk of aluminum plating, especially in connection with large diesel engines that use aluminum pistons.
Cast metal piston rings have open graphite lamellae on their flank surfaces. A lubricating effect is achieved in this way so the problem of plating or microfusion occurs less often on piston rings made of cast metal.
In order to prevent plating on piston rings employed in Otto engines, the flank surfaces are often provided with various break-in coatings. For example, German patent DE 35 02 143 C3 shows break-in coatings made from manganese phosphate, zinc phosphate or tri-iron-tetraoxide (Fe.sub.3 O.sub.4) for nitrated steel piston rings. The outermost layer, which is referred to as the composite layer, is removed, and one of the oxide layers is directly applied to the diffusion layer disposed underneath. This, however, requires additional work since the composite layer to be removed is extremely hard due to the iron nitrate formed therein.
Substantially higher combustion pressures develop in Diesel engines than in Otto engines, which increases the wear on the flank surfaces of the piston ring. For this reason, a ring carrier is installed to prevent groove wear, especially with aluminum pistons used in diesel engines. This ring carrier usually consists of austenitic cast iron. While this procedure avoids aluminum plating, it is very expensive.
Attempts have been made to avoid aluminum plating through the use of controlled surface roughness. However, the effectiveness of this procedure has not been reliably determined. The rings are manufactured with plateaus projecting from the surface of the flank in order to form a lubricant reservoir. However, rough wear will nonetheless occur, which will then impermissibly change the piston ring groove through abrasion.